Stainless Steel Coil Tube (盘管) for Fan Coil Unit Applications
Manufacturer: Wenzhou Jixing Metal Materials Co., Ltd. | Product No.: 445 | Classification: Literature Study / Technical Documentation
1. Product Overview
The stainless steel coil tube (盘管, Coiltube) is a precision-engineered tubular product specifically designed for integration into fan coil unit (FCU, 风机盘管) systems and related hydronic air distribution assemblies. As a critical heat exchange component, the coil tube serves as the primary medium for thermal energy transfer between circulating fluid (typically chilled water or hot water) and the surrounding airstream within HVAC (Heating, Ventilation, and Air Conditioning) installations.
Wenzhou Jixing Metal Materials Co., Ltd. produces coil tubes that meet stringent dimensional tolerances and mechanical property requirements, ensuring optimal aerodynamic and thermal performance within fan coil unit assemblies. The coil tube geometry — characterized by a continuous helical winding of seamless or welded stainless steel tubing — directly influences the overall aerodynamic response of the FCU system, including static pressure characteristics, airflow distribution uniformity, and pressure drop across the heat exchanger core.
Academic Context: Research on multi-wing centrifugal fan group optimization design has demonstrated that coil tube configuration parameters — including tube diameter, coil pitch, number of passes, and fin spacing — exert a measurable and quantifiable response on the integrated aerodynamic performance of fan coil units. This product is engineered to support such optimization studies and to deliver predictable performance within optimized FCU architectures.
2. Material & Properties
2.1 Base Material
The coil tube is manufactured from austenitic stainless steel, with the following grades being standard:
- AISI 304 (UNS S30408): The primary grade for general-purpose FCU coil tubes. Contains approximately 18% Chromium (Cr) and 8% Nickel (Ni), providing excellent corrosion resistance in indoor hydronic environments and adequate thermal conductivity.
- AISI 316 (UNS S31608): Enhanced grade incorporating 2–3% Molybdenum (Mo) for superior resistance to chloride-induced pitting corrosion, suitable for coastal or chemically aggressive applications.
- AISI 321 (UNS S32108): Titanium (Ti) stabilized variant, recommended for applications involving sustained operating temperatures above 425°C where intergranular corrosion resistance is critical.
2.2 Key Physical & Mechanical Properties (AISI 304 Reference)
| Property | Symbol | Value / Range | Unit |
|---|---|---|---|
| Density | ρ | 8.00 | g/cm³ |
| Thermal Conductivity (at 20°C) | k | 16.3 | W/(m·K) |
| Thermal Conductivity (at 100°C) | k | 18.6 | W/(m·K) |
| Specific Heat Capacity | cp | 500 | J/(kg·K) |
| Modulus of Elasticity | E | 193 | GPa |
| Yield Strength (0.2% offset) | σy | ≥ 205 | MPa |
| Tensile Strength (Rm) | — | 515 – 730 | MPa |
| Elongation (A50) | — | ≥ 40 | % |
| Hardness (Rockwell B) | HRB | 80 – 95 | — |
| Maximum Service Temperature | Tmax | 870 | °C |
2.3 Corrosion Resistance
The austenitic stainless steel coil tube exhibits a passive oxide film (Fe2O3 / Cr2O3 composite layer) that self-repairs in oxygen-containing environments. This passive layer provides:
- Resistance to atmospheric corrosion (indoor HVAC environments)
- Resistance to dilute aqueous solutions of common hydronic treatment chemicals
- Resistance to organic acids and food-grade sanitizing agents (relevant for cleanroom HVAC)
- Resistance to condensation-induced corrosion on the exterior tube surface
3. Specifications
3.1 Dimensional Specifications
| Parameter | Specification | Tolerance | Standard Reference |
|---|---|---|---|
| Outer Diameter (OD) | 6.35 mm (¼"), 9.53 mm (⅜"), 12.70 mm (½"), 15.88 mm (⅝") | ±0.05 mm | ASTM A269 / A213 |
| Wall Thickness | 0.40 mm – 1.00 mm (typical: 0.60 mm) | ±0.03 mm | ASTM A269 |
| Coil Pitch (Axial Spacing) | 15.0 mm – 30.0 mm | ±0.5 mm | Internal Specification |
| Number of Coil Rows (Passes) | 2 / 4 / 6 / 8 rows (configurable) | — | Per FCU Design Requirement |
| Tube Length per Row | 300 mm – 1500 mm | ±1.0 mm | Internal Specification |
| Fin Thickness (if finned) | 0.10 mm – 0.20 mm (aluminum fins) | ±0.02 mm | ASHRAE Standard 34 |
| Fin Spacing | 1.5 mm – 3.0 mm | ±0.1 mm | ASHRAE Standard 34 |
| Overall Coil Dimensions (W × H × D) | Custom (typical: 500 × 300 × 150 mm) | ±2.0 mm | Per Application |
| End Connection Type | Flanged / Threaded (G1/2", G3/4") / Brazed | — | EN 10226 / BS 3789 |
| Surface Finish | 2B (cold-rolled, bright annealed) / No. 4 Brushed | Ra ≤ 0.8 μm | ASTM A240 |
3.2 Performance Specifications
| Parameter | Value / Range | Test Condition |
|---|---|---|
| Rated Operating Pressure (Design) | 1.0 MPa (10 bar) | Hydrostatic Test at 1.5× Design Pressure |
| Operating Temperature Range | −40°C to +150°C | Continuous Service |
| Heat Transfer Coefficient (U-value) | 35 – 55 W/(m²·K) | Chilled Water / Air Side, Finned Configuration |
| Pressure Drop (Water Side) | ≤ 25 kPa (per row, at 1.0 L/s) | ASTM F436 Method |
| Pressure Drop (Air Side) | 15 – 45 Pa | Depends on Fan Configuration & Fin Density |
| Service Life (Design) | ≥ 20 years | Indoor Hydronic Application |
4. Manufacturing Process
4.1 Process Flow
- Raw Material Procurement & Inspection: Stainless steel seamless tubes (per ASTM A269) or welded tubes (per ASTM A213) are procured from certified mills. Incoming inspection includes Chemical Composition Analysis (CMA) via Optical Emission Spectroscopy (OES), mechanical property verification, and dimensional measurement.
- Tube Cutting & Deburring: Precision saw cutting to specified row lengths, followed by mechanical deburring of internal and external edges to eliminate flow restriction and stress concentration sites.
- Coil Forming (Bending): Cold bending of tubes into the helical coil geometry using CNC-controlled bending machines. The bending process is executed under controlled conditions to avoid work-hardening-induced embrittlement and to maintain wall thickness uniformity within tolerance.
- U-Bend Formation: U-shaped return bends (hairpin bends) are formed at row transitions to connect adjacent tube rows. Minimum bend radius is maintained at ≥ 3× tube OD to prevent wall thinning and cracking.
- Fin Insertion (Fin Coiling): For finned coil configurations, aluminum fins (typically 1100 or 3003 grade) are slit and expanded around the coil tube using a fin-coiling machine. This creates a continuous fin-and-tube heat exchanger core.
- End Connection Fabrication: Header pipes (flow and return) are welded or brazed to the coil tube ends, with connection flanges or threaded fittings attached per the specified connection type.
- Welding & Brazing: Where welding is required, TIG (Tungsten Inert Gas) welding is performed under argon shielding to prevent oxidation of the austenitic stainless steel. Brazing joints (where applicable) use silver-based filler alloys (e.g., Cu-Ag-5) with maximum service temperature of 600°C.
- Pickling & Passivation: Post-welding treatment with nitric acid (HNO3) / hydrofluoric acid (HF) pickling solution to remove heat-affected zone discoloration, followed by passivation treatment to restore the chromium oxide passive film (per ASTM A967).
- Hydrostatic Pressure Testing: Each coil tube assembly is pressure-tested at 1.5× the design operating pressure (typically 1.5 MPa) for a minimum hold time of 30 minutes, with zero leakage permitted.
- Final Inspection & Packaging: Visual inspection, dimensional verification, and documentation compilation. Coils are packaged in protective wrapping with moisture-absorbing desiccant for shipment.
4.2 Critical Process Control Points
- Bend Radius Control: Minimum bend radius of 3D (D = tube outer diameter) to prevent ovality exceeding 5% and wall thinning exceeding 15%.
- Weld Quality: Full-penetration TIG welds with visual inspection per AWS D10.9; radiographic testing (RT) available for critical joints.
- Fin-Tube Bonding: Fin insertion depth and crimp pressure verified to ensure thermal contact resistance is minimized and mechanical fixation is adequate for thermal cycling.
- Passivation Verification: Iron contamination test (ASTM A967 Method A) performed on all passivated surfaces to confirm absence of free iron.
5. Applications
5.1 Primary Application: Fan Coil Units (FCU)
The stainless steel coil tube is the core heat exchange element in fan coil units, which are terminal air handling devices used in commercial, industrial, and institutional HVAC systems. The coil tube transfers thermal energy between the circulating hydronic fluid and the airstream drawn through the unit by a centrifugal or axial fan.
5.2 Specific Application Domains
- Commercial Building HVAC: Office buildings, hotels, hospitals, shopping centers — where indoor air quality, corrosion resistance, and hygienic performance are paramount.
- Cleanroom Systems: Pharmaceutical manufacturing, semiconductor fabrication, and laboratory environments requiring non-shedding, non-corrosive heat exchange surfaces.
- Food Processing & Dairy Industry: Sanitation-critical environments where CIP (Clean-in-Place) compatibility and resistance to organic acid corrosion are required.
- Marine & Coastal Applications: Ship HVAC systems and coastal facility installations where chloride-induced corrosion is a design concern (AISI 316 grade recommended).
- Data Center Cooling: Precision temperature control in server rooms, where reliability and long service life are critical.
- Residential High-End HVAC: Ductless mini-split systems and premium residential air handling units.
5.3 Integration with Multi-Wing Centrifugal Fan Systems
Based on research into multi-wing centrifugal fan group optimization design (多翼离心风机分组优化设计), the coil tube configuration directly responds to and influences the aerodynamic performance of the integrated FCU system. Key interaction parameters include:
- Air-side pressure drop response: Coil tube row count, fin density, and tube pitch determine the static pressure rise required from the centrifugal fan group.
- Airflow distribution uniformity: Coil geometry affects the spatial distribution of airflow velocity across the coil face, impacting the fan group's operating point on its performance curve.
- Acoustic performance: The aerodynamic noise generated at the coil-tube/fin interfaces contributes to the overall sound pressure level (SPL) of the FCU system.
- Optimization response: Parametric studies demonstrate that coil tube configuration variables can be optimized to minimize fan power consumption while maintaining required heat transfer capacity, with response sensitivities quantified through computational fluid dynamics (CFD) modeling.
6. Quality Certifications & Standards Compliance
6.1 Applicable Standards
| Standard | Title |
|---|